Polyethylene is the most widely used commercial polymer. It can generally be prepared by a couple of different processes. Polymerization in the presence of free-radical initiators at elevated pressures was the method first discovered to obtain polyethylene and continues to be a valued process with high commercial relevance for the preparation of low density polyethylene (LDPE).
A normal set-up of a plant for preparing low density polyethylene comprises a polymerization reactor which can be an autoclave or a tubular reactor or a combination of such reactors as well as further equipment. For pressurizing the reaction components, a set of two compressors, a primary and a secondary compressor, is often used. At the end of the polymerization sequence, a high-pressure polymerization unit may include apparatuses like extruders and granulators for pelletizing the resulting polymer. Furthermore, such a polymerization unit may also comprise means for feeding monomers and comonomers, free-radical initiators, modifiers or other substances at one or more positions to the polymerization reaction.
A characteristic of the radical initiated polymerization of ethylenically unsaturated monomers under high pressure is that the conversion of the monomers is often incomplete. For each pass of the reactor, only about 10% to 50% of the dosed monomers are converted in the case of a polymerization in a tubular reactor, and from 8% to 30% of the dosed monomers are converted in case of a polymerization in an autoclave reactor. Accordingly, it is common practice to separate the discharged reaction mixture into polymeric and gaseous components and recycle the monomers. To avoid unnecessary decompression and compression steps, the separation into polymeric and gaseous components is usually carried out in two stages. The monomer-polymer mixture leaving the reactor is transferred to a first separating vessel, frequently called a high-pressure product separator, in which the separation in polymeric and gaseous components is carried out at a pressure that allows for recycling of ethylene and comonomers separated from the monomer-polymer mixture to the reaction mixture at a position between the primary compressor and the secondary compressor. At the operating conditions of the first separation vessel, the polymeric components within the separating vessel are in a liquid state. The level of the liquid phase in the first separating vessel is generally measured by radiometric level measurements and is controlled automatically by a product discharge valve. The liquid phase obtained in the first separating vessel is transferred to a second separation vessel, frequently called a low-pressure product separator, in which a further separation in polymeric and gaseous components takes place at lower pressure. The ethylene and comonomers separated from the mixture in the second separation vessel are fed to the primary compressor, where they are compressed to the pressure of the fresh ethylene feed, combined with the fresh ethylene feed, and the joined streams are further pressurized to the pressure of the high-pressure gas recycle stream.
Measuring the filling level within the first separating vessel is usually carried out by a radiometric level measurement system based on gamma radiation, because such systems are very reliable for extreme processing conditions. The operation principle is based on the properties of gamma rays, which lose intensity as they pass through material(s). Attenuation of the gamma radiation when passing through the vessel is measured by a detector. The intensity of the passing radiation is predictably affected by the type of the material, the density of the material and the total thickness of the object, and by the distance between the gamma ray source and the detector. In the case of two phases of different density inside a vessel, the extent to which the radiation is attenuated is also dependent on the proportion of the two phases in the path of radiation, i.e. on the filling level within the vessel, if one phase is in a gaseous state and the other phase is in a liquid state. High reliability and low maintenance costs of the radiometric measurement systems, even under harsh operating conditions, have been established. Usually the gamma rays used for level measurement are generated by nuclear gamma decay. The penetrating power of the radiation is characterized by its photon energy, expressed in electron volts (eV), which relates to the wavelength of the gamma radiation. As any radioactive isotope decays, the intensity of a gamma source decreases in correspondence to the half-life time of the utilized radioisotope. The most common isotope used for generating gamma radiation for level measurement is cesium-137, which has a photon energy level of 0.66 MeV. Another suitable isotope is cobalt-60, which has an energy level of 1.3 MeV. While the greater penetrating power of this higher energy radiation offers an advantage, the drawback is that cobalt-60 has a shorter half-life time. For measuring the gamma radiation which has passed the separation vessel, different kinds of radiation detectors can be used. Suitable gamma radiation detectors are, for example, ionization chambers, Geiger-Müller tubes or scintillation counter detectors.
A difficulty in ensuring an accurate level of measurement comes from the significant variation in process gas density and composition if different polyethylene grades, especially with significantly different comonomer content, are produced in the same high-pressure production line. In such a case, not only can the density of the gaseous fraction within the separation vessel vary over a very broad range but also the difference between the density of the gaseous fraction and the density of the liquid fraction can become relatively small. In addition, fluctuations of gas properties might simulate level changes which in reality do not exist because an increasing gas density might be interpreted as an increase in the filling level. Fluctuating gas properties accordingly affect the accuracy of the level measurement.
In addition, an accurate level measurement has an impact on the safety of the polymerization process and the consequences if undesired process conditions occur. Pressure apparatuses used as the first separating vessel in a high-pressure polymerization process are usually equipped with an emergency pressure release system comprising an emergency pressure release valve and one or more bursting discs. If the level of the liquid fraction in the separation vessel decreases below a pre-defined minimum value, or rises above a pre-defined maximum value, the polymerization process has to be interrupted. In such a case, a controlled emergency program should be initiated. If such an emergency program is not triggered, an emergency release of hydrocarbons via failing bursting discs to a safe location might be caused. Exchange of the bursting disks on the high pressure equipment will then be necessary, which normally leads to a relatively long plant shut down and loss of production.
New modern world scale plants are designed with a much higher capacity of a single production line than in the past. This results in much stricter requirements for the reliability of the plant because of a smaller number of lines and higher production rates. Furthermore, due to the higher throughput, it is necessary to use separating vessels of increased diameters, and accordingly the measuring distances for the radiometric measurement increases. Moreover, it is advantageous if the service life of the utilized radioactive sources of the decay related decrease in radiation intensity is not too short.
An object of the present disclosure is to overcome the disadvantages of the prior art and provide an improved level measurement system in the first separation vessel of a high-pressure polymerization of ethylenically unsaturated monomers that gives an accurate measurement of the filling level of the liquid fraction in the separation vessel. In addition, the process should allow for a fast grade change between different types of produced low density polyethylenes. It should further allow for operating nuclear radiation sources with a longer service life, and fulfilling the requirements in larger scale polymerization plants and with the production of different polymer grades of different comonomer content and produced under different polymerization conditions.